1. Field of the Invention
The present invention relates to a light intensity-modulated direct overwrite-capable magneto-optical recording medium.
2. Prior Art
In a magneto-optical (MO) recording medium, recording (or writing) is carried out by locally heating a magnetic thin-film using laser light, for example, and reversing the direction of magnetization in this heated area with an external magnetic field. These recorded domains with their differing directions of magnetization are read out by means of the Kerr effect and the Faraday effect.
Advantages of magneto-optical recording media are their high recording density and also, unlike hard disks (a high-capacity magnetic recording medium), the easy replaceability of the medium. However, conventional magneto-optical recording media are generally not directly overwritable during rewriting. Because new information must be recorded after first erasing the recorded information, the rewriting process has been slow.
Magneto-optical recording media which are directly overwritable by light intensity modulation (sometimes referred to hereinafter as "light-modulated overwriting") are described in JP-A 175948/1987, JP-B 16993/1996, JP-B 16996/1996 and elsewhere. However, these magneto-optical recording media require that the drive be provided with an initializing magnet. Magneto-optical recording media which are capable of light-modulation overwriting without requiring an initializing magnet are described in, for example, International Publication WO 90/02400, JP No. 2503708, and JP-A 12711/1994.
A problem with light-modulated overwrite-capable magneto-optical recording media is that, as will be discussed subsequently, because two levels of recording power are used in overwriting, the recording power margin is generally narrow, and the power margin during low-power recording is especially narrow. "Recording power margin," as used herein, refers to the recording power range (breadth) at which a sufficient carrier-to-noise ratio (CNR) can be obtained. For example, if the laser power during low-power recording is too low, there is substantially no formation of record marks and incomplete erasure occurs, as a result of which the CNR becomes very low and errors increase. On the other hand, if the laser power during low-power recording is too high, an effect similar to that during high-power recording arises, as a result of which normal formation of record marks for low-power recording does not occur, likewise leading to a considerable drop in the CNR and an increase in errors.
In magneto-optical recording media drives, the laser power applied to the medium fluctuates due to such factors as individual variations in the semiconductor lasers used in optical pickups, changes with time in the semiconductor laser, and contamination of the optical pickup optics. Thus, if the medium becomes standardized at a narrow recording power margin, the allowable range in the laser power fluctuation of the drive will narrow, making it difficult to achieve lower-cost drives.
Moreover, in light-modulated overwritable magneto-optical recording media, along with increasing the recording power margin during low-power recording, there is also a need to prevent a rise in the recording power required during high-power recording.
JP-A 106744/1992 and JP-A 234158/1993 describe how to increase the recording power margin in light-modulated overwritable magneto-optical recording media.
In JP-A 106744/1992, a thermal diffusion layer composed of a metal material having a high heat conductivity is provided, either in direct contact with two magnetic thin-films or with intervening protective layers composed of a dielectric. In this reference, the thickness of the protective layers has been set at not more than 50 nm and the thickness of the thermal diffusion layer at from 2 to 50 nm. In an example provided therein, 10 nm thick protective layers made of SiN and a 20 nm thick thermal diffusion layer made of copper are formed.
JP-A 234158/1993 discloses, as in JP-A 106744/1992 above, a magneto-optical recording medium with a thermal diffusion layer, in which there has been provided, between the thermal diffusion layer and a magnetic thin-film, a thermal insulating layer composed of a dielectric. In this reference, from two to four magnetic thin-films are stacked together, and the thickness of the thermal insulating layer is from 2 to 10 times the thickness of the thermal diffusion layer. An example provided therein describes the formation, on two magnetic thin-films, of an 80 nm thick thermal insulating layer made of SiO.sub.2 and a 40 nm thick thermal diffusion layer made of aluminum.
However, we found through our own research that, even when a dielectric layer (as a protective layer or a thermal insulating layer) and a thermal diffusion layer are provided as in above-mentioned JP-A 106744/1992 and JP-A 234158/1993, it is not possible to increase the recording power margin and to avoid a rise in the recording power during high-power recording.
Problems of prior-art magneto-optical recording media which do not require an initializing magnet and are light-modulation overwritable, such as the magneto-optical recording medium described in JP-A 12711/1994, are that a sufficiently high CNR cannot be obtained during overwriting, and that the CNR deteriorates with repeated overwriting.